Lee Sun Hee's Fox Rain from My Girlfriend is a Gumiho is one of the most beautiful OST's I've ever heard, it's my favourite.
I don't think I have a favorite drama OST, but I have a lot of favorite songs from dramas:

Tamra, The Island: Your Tears (Main Theme), Jane Park - 이어사나 and 미카 - Blue (Love Theme)

Gumiho: Tale of The Fox's Child: 상사 (모래) and Opening Title

White Christmas: Opening Title

City Hunter: Oh Jun Sung - City Hunter (instrumental)

Cruel City: Jo Jung Hee (조정희) - Everyday

The Princess Man: Finally (instrumental) and Destino

The Devil: Opening Title

Ghost: Block B - Burn Out

Iljimae:Park Hyo Shin (박효신) - 화신

Goong: Opening Title and Frozen Pond

Hahaha might continue my list later. xD
Secret Garden, That man <3

Heartstrings, Because I miss you <3

Gu Family Book, Last Words <3

You're Beautiful, Without Words <3

With lots of others actually, drama's osts are so good!
All of Coffee Prince's OST songs
Cinderellas's Sister Yesung - It has to be you
Secret Garden Baek Ji Young - That woman
Gu Family Book 4men - Only You
You are Beautiful Jang Geun Suk - Without words
Arang and the Magistrate K.Will - 사랑은 그대다
Boys over Flowers Jisun - What should I do
Heartstrings Yonghwa - Because I miss you
Cruel City 김용진 (보헤미안) [Kim Yong Jin of Bohemian]- Hurt
I can't really choose the top one so I'll just give the list of some of them.
Goong: Perhaps Love, Parrot, and You and I are Fools
City Hunter: Nana's Theme
Rooftop Prince: Hurt, After a Long Time
Queen In Hyun's Man: I'm Going to Meet You
My Girlfriend is a Gumiho: Fox Rain
Shining Inheritance: Crazy in Love
Secret Garden: Guardian Angel
Dream High: Love High
Secret love: incurable disease Cheongdamdong alice: daddy long legs Dream high: all the songs
That Winter the Wind Blows Mary Stayed Out All Night Gentleman's Dignity Love Rain You're Beautiful Dream High
The Moon That Embraces the Sun Boys Over Flowers Cheongdamdong Alice Goddess of Marriage Dream High Heading to the Ground That Winter That Wind Blows
Shut up: Flower boy band You're beautiful Heirs Mary stayed out all night Dream High City Hunter Secret Garden - that man Queen In Hyun's Man Heartstrings
City Hunter 1 Litre of Tears A Love To Kill My Girl Love Rain Lie to Me Two Weeks Delightful Girl, Choon Hyang Coffee Prince Boys Before Flowers
Ok currently obsessed with all of secret love ost
for me it would be: secret garden ost (Liar by Oska is love and also that man by Hyun bin) innocent man ost (love is like snow) fugitive plan b ost (fav song from there would be Mayday)
For me 1 ) Iljimae' s ost because it is unique . 2) Chuno 's ost : such a masterpiece aigoo . You all, should listen to it . Then : The princess' man , Warrior Baek Dong Soo , My girl , Time between dog & wolf , The master's Sun , Shut up flower boy band etc .
For me it is Bridal Mask/Gaksital and Secret (2013). They have the best OST songs and V.A.

But all music dramas has good OST songs too, but I think those are the best. :)
Because Im stupid - Boys before flowers That man - Hyun Bin Touch Love - Master's sun Love is the moment - Heirs Crying again - Heirs